Overview of Hurricanes
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Transcript Overview of Hurricanes
Carl Schreck
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Winds < 35 kts
Numbered, but not
named
Semi-organized group of
thunderstorms
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Winds 35–63 kts
≈ 11 per year in the
Atlantic
Named
More circular/spiral, but
no eye
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Winds ≥ 64 kts
≈ 6 per year in the Atlantic
Eye develops
Different names around the
world:
Hurricane in the Atlantic and
East Pacific
Typhoon in West Pacific
Cyclone in Indian Ocean and
South Pacific
“Tropical Cyclone” is the
generic/scientific term
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Winds ≥ 96 kts
≈ 2-3 per year in the
Atlantic
Category 3–5 on the
Saffir–Simpson Scale
Strong and dangerous
storms
Well-defined, circular eye
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Deep warm ocean layer
Warm oceans are the energy
source for hurricanes
But hurricanes can mix cold
water upward if the warm
water is too shallow
Conditionally unstable
atmosphere
Warm air rises, but only if it’s
warmer than its surroundings
Moist mid-troposphere
Dry air 2-3 km from the
surface can cause cold
downdrafts
ww2010.atmos.uiuc.edu
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Pre-existing convection
Needs organized
thunderstorms to get things
going
Cyclonic low-level vorticity
Counter-clockwise spinning
winds help organize the
thunderstorms
Associated with low pressure
Weak vertical wind shear
Tilting winds can knock the
storm over
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Wind shear is defined as the wind vector
difference between the 850 and 200 mb
level (arbitrary)
Good – latent
heat can
concentrate in
one area
Bad – convection
torn apart
High westerly shear
Low easterly shear
In general, low values (< 20 kt) of vertical wind
shear are desired.
Courtesy of C.C. Hennon, UNC Asheville
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Berry et al. (2007, Mon. Wea. Rev.)
Thunderstorm systems with
wave-like properties
Move westward with a period
of 3–6 days
Common over West Africa
Provides storms with cyclonic
vorticity and convection
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Warm Ocean Temperatures
10°N to 20°N
Supply of Tropical Waves
from Africa
20°W to 60°W or 80°W
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Zipser et al. (2009, BAMS)
climate.nasa.gov
Dry air from Sahara can
weaken storms
Dust blocks the sun and
cools the ocean
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Knapp et al. (2010, BAMS)
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Bars depict number
of named systems
(open/yellow),
hurricanes
(hatched/green),
and category 3 or
greater (solid/red),
1886-2004
nhc.noaa.gov/climo
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Historical record of
tropical cyclone locations
and intensities
Constructed after each
season by the warning
agencies
Best estimate using all
available data
nhc.noaa.gov
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Weather stations
Buoys
Ships
Radar
Aircraft
Reconnaissance
Visible/Infrared
Satellites
Microwave satellites
Satellite estimates
account for vast
majority of the
record
Courtesy of Jack Beven (NOAA/NHC)
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Weather stations
Buoys
Ships
Radar
Aircraft
Reconnaissance
Visible/Infrared
Satellites
Microwave satellites
Satellite estimates
account for vast
majority of the
record
Courtesy of Jack Beven (NOAA/NHC)
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Which image on the right is the
closest match for this image of
Hurricane Katrina?
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